1. Field of the Invention
This invention relates to an optical integrated voltage sensor for measuring the magnitude of a voltage in an optical manner.
2. Description of the Prior Art
In today's highly information-orientated world, the importance of electromagnetic waves as a medium for propagating information for broadcasting and communications is growing, and both the intensity and frequency of these waves are tending to increase.
At the same time, the problem of functional malfunctions in electronic equipment, caused by these electromagnetic waves, is also increasing. There is a danger that such functional malfunctions in equipment could cause a great deal of damage, particularly in the fields of medical equipment which is indispensable for maintaining human life, office equipment which processes large quantities of information at speed, and automobile electronics. That is why measures to counteract malfunctions due to electromagnetic waves have become an extremely important topic of technical concern.
When it comes to evaluating the resistance of electronic equipment to malfunctions due to electromagnetic waves, or when countermeasures are necessary after a functional error has occurred as the result of a malfunction due to electromagnetic waves, it is necessary to place the electronic equipment in an electromagnetic environment that re-creates that state, measure the voltage signal waveforms that are output from the circuitry within the electronic equipment, and thus determine the source of abnormal signals. An oscilloscope is well-known as means of monitoring such voltage signal waveforms.
However, cables that are metal conductors are used in this method for transferring the output signals from measurement locations within the circuitry to the input terminals of the oscilloscope. This means that noise is induced by electromagnetic fields within this metal cable, making accurate measurement impossible.
In order to solve these problems, voltage sensors that are being developed use optical integrated circuits formed on a substrate of an electro-optical crystal, such as LiNbO3, within the voltage detector sections thereof.
An example of such a voltage sensor is shown in FIG. 2.
This sensor comprises a light source 10, a light detector 12, and a sensor section 21 configured of an optical integrated circuit 32 that functions as a waveguide type of branching interferometer optical modulator. These components are connected together optically by an optical fiber 11 for inputting a measuring beam and another optical fiber 13 for outputting the measuring beam. The optical integrated circuit 32 has a waveguide 17 for propagating the measuring beam that is incident thereto through the optical fiber 11. This waveguide 17 is configured to branch into first and second modulation-inducing waveguides 15a and 15b at a branch section 23, then combine these waveguides once again at a wave-combining section 25.
Modulation-inducing electrodes 34a and 34b are formed on the first and second modulation-inducing waveguides 15a and 15b. When fine metal wires 27a and 27b that extend therefrom are brought into contact with an object 18 whose voltage is to be measured, which acts as a voltage measurement section within an electrical circuit, the voltage of the object 18 whose voltage is to be measured is applied to the first and second modulation-inducing waveguides 15a and 15b as voltage signals of opposite signs, through the modulation-inducing electrodes 34a and 34b.
When a measuring beam from the light source 10 is input towards the sensor section 21 through the optical fiber 11, this measuring beam is propagated into the optical modulation section through the waveguide 17 in the optical integrated circuit 32.
As described previously, voltage signals of opposite signs are applied to the modulation-inducing electrodes 34a and 34b in the first and second modulation-inducing waveguides 15a and 15b, so that a phase difference is generated in the light waves travelling within the waveguides 15a and 15b. Therefore, constructive interference at the wave-combining section 25 between the light waves passing through the two waveguides 15a and 15b causes the intensity of the light to be modulated by the phase difference, then this light is input to the light detector 12 through the optical fiber 13.
In this particular case, the voltage required for varying the luminous energy from a maximum to a minimum, which is called the half-wavelength voltage, is no more than a few volts. Thus, since a large change in luminous energy is achieved even when a small voltage is applied, the measurement sensitivity of this sensor is extremely high.
It is therefore possible for this signal processing section 20 to obtain the phase difference from the intensity of the modulated measuring beam, and thus the magnitude of the voltage, by using the light detector 12 to measure the intensity of the modulated measuring beam that is output from the optical integrated circuit 32 and inputting the result to the signal processing section 20.
A particular characteristic of this type of device is the way in which components such as the sensor section 21 and the optical fibers 11 and 13 are mainly formed by using dielectric materials. This makes it possible to measure voltages accurately and with a high degree of sensitivity, without any effects due to electromagnetic fields.
However, two optical fibers 11 and 13 are connected to this device, for inputting a measuring beam to the optical integrated circuit 32 and outputting it therefrom, as described above. It is therefore impossible to make the sensor section 21 compact enough for practicable use, raising the problem that it is difficult to measure voltages in cramped locations.
In other words, it is desirable to design the sensor section used in such a device to have a cantilevered structure, so that one of the two optical fibers 11 and 13 shown in FIG. 2 must be curved and connected to the sensor section. However, optical fibers cannot be said to be very flexible, so their minimum radius of curvature is on the order of several cm. This raises a problem when forming a cantilevered-structure sensor in that it is difficult to make it compact enough to meet the practicability requirements of a size of no more than 20 to 30 mm.
Furthermore, a connection portion between an optical fiber and a waveguide requires the most precise adjustment and is subject to mechanical slippage due to factors such as temperature or vibration, and thus changes in the characteristics thereof can easily occur. Since the sensor shown in FIG. 2 requires two optical fibers, it has to have two such connections. Making these connections is an extremely awkward task and this has prevented the inexpensive fabrication of a device that can operate stably.
This device has a further problem in that, if the voltage applied to the modulation-inducing electrodes should exceed the dielectric breakdown voltage of the optical integrated circuit substrate and modulation-inducing electrodes, even accidentally, the optical integrated circuit substrate and modulation-inducing electrodes could be damaged by dielectric breakdown, making the device unusable.